In recent years, LTE (Long Term Evolution) has been investigated in 3GPP (3rd Generation Partnership Project) as long-term evolution of a next generation access method. In LTE, the Orthogonal Frequency Division Multiplexing (OFDM) modulation method, which utilizes the orthogonality of frequency, is likely to be employed, in order to increase the efficiency of frequency usage. As an access method there, adoption of the Orthogonal Frequency Division Multiple Access (OFDMA) utilizing OFDM has been investigated. Being different from single carrier-wave systems using single carrier-wave such as the Code Division Multiple Access (CDMA), OFDMA is a multiple carrier-wave system in which the information is transmitted on a carrier wave divided into a plurality of subcarriers each having a narrow frequency bandwidth.
In OFDMA, using its feature that a real-time allocation of wireless resources is possible, a “resource scheduling” has been investigated, in which a “resource block”, as a user resource, is flexibly assigned in each of the time domain and the frequency domain.
A method of controlling the assignment of a PDSCH (Physical Downlink Shared Channel) consisting of downward (downlink) data channels in a transmission signal, every unit time of 1 msec called “sub-frame”, can be exemplified as a specific example of a resource scheduling in OFDMA in LTE.
Accordingly, in the resource scheduling in LTE, there is a sub-frame to which no PDSCH (Physical Downlink Shared Channel) is assigned, and such a sub-frame is in the state where only a reference signal for piloting is transmitted. The reference signal is transmitted only during a predetermined short period of time within a period for the sub-frame.
When there is a sub-frame which has no PDSCH assigned and accordingly has only a reference signal, the transmission signal becomes intermittent.
In a transmission apparatus, for example, in a system where wireless resources are assigned intermittently, as mentioned above, when a transmission power amplifier is operated continuously, there arises a problem of wasteful power consumption even in a time period where no wireless resource is assigned, which is due to an idling bias of an amplification device. This problem has not been revealed up to now in the case of a continuous single carrier-wave system such as CDMA. However, in the case of applying the resource scheduling in OFDMA, when the ratio of the time period with no resource assignment to the whole time period (the intermittence rate) is large, operational power efficiency averaged over the whole sub-frames declines.
In general, with respect to power consumption reduction of a transmission apparatus at a base station for mobile communication, the technical investigation is conducted not only on the reduction in power consumption at the time of maximum transmission power but also on that in intermittent operation and at standby, from the aspect of reduction of operating expenditure (OpEx) of the system.
There are various technologies for reducing the power consumption in a communication apparatus (Patent Documents 1 and 2 are referred to, for example). In the technologies of Patent Documents 1 and 2, the power consumption at the time of low input level is reduced by changing the bias of a power amplifier in accordance with the transmission level.
There is also a technology in which the power consumption of a power amplifier for OFDM signal amplification is reduced (Patent Document 3 is referred to, for example). The power amplifier of Patent Document 3 delays an OFDM signal, which is subjected to the amplification, by a predetermined period of time, during which the power amplifier judges whether the signal level has exceeded a certain level or not. Based upon a result of the judgment, the drain voltage or bias voltage of the power amplifier is controlled.